1. Field of the Invention
The present invention relates to an excavator as an excavation machine using an electric motor as a driving source.
2. Discussion of the Background
In the past, a general excavator called a hydraulic excavator is constituted so that an upper turning body is mounted on a lower traveling body (normally, a crawler), and an excavation attachment provided with a boom, an arm and a bucket is mounted on the upper turning body, whereby a hydraulic pump is rotated by an engine, as a driving source, installed on the upper turning body to supply its discharge oil to a hydraulic actuator (a hydraulic motor or a hydraulic cylinder) to effect traveling, turning, boom rising and falling, arm operation, and bucket operation.
However, the aforementioned conventional hydraulic type excavator has a problem that a burden applied to the engine is great, fuel cost is high, and exhaust gases and noises are involved.
In view of the above, there has been proposed a so-called hybrid type in which an electric motor is rotated by an engine, and a hydraulic pump is driven by the electric motor, as disclosed in Japanese Utility Model Application Laid-Open No. Hei 5 (1993)-4850 Publication.
The above-described well known hybrid type excavator is designed so that when the pump load is light, the electric motor is allowed to function as a generator by surplus torque of the engine (regenerative function) to store power in a battery, whereas at the time of heavy load, the engine is assisted by the stored power of the battery, thus smoothing the engine load, reducing the exhaust gases an reducing the fuel cost.
However, according to the above-described well known hybrid type excavator, since the turning force is transmitted through a channel of the enginexe2x80x94electric motorxe2x80x94hydraulic pump, it is necessary to mechanically connect these three elements, and collecting the latter at one place as a single power unit and installing them.
This poses the following disadvantages:
(1) The power unit is large, and a large space is required for the installing part (upper turning body), these points impede the arrangement of devices, which is unsuitable for a small turning type excavator which recently becoming the main current.
(2) The variation of load of the excavator is particularly sever in terms of time and level as compared with an automobile or the like so that switching of the mechanical connecting parts of the power unit is frequently carried out, thus posing a great problem in terms of responsiveness and durability of the power unit.
(3) As a problem in terms of oil pressure, the following occur:
(a) The distance between the hydraulic pump of the power unit and the hydraulic actuator installed at each operating part of the excavator is so long that pressure loss is great;
(b) Energy loss particularly at the time of fine operation is great due to the bleed off from a control valve (a switching valve) provided between the pump and the actuator; and
(c) A pilot hydraulic source (a pump) for the pilot control of the control valve is also necessary.
As described, the energy loss of the hydraulic system is large, and the actual effect of improving fuel cost is low.
Thus, the present invention provides an excavator in which a power unit can be installed by being divided and scattered at a small unit, frequent switching of the mechanical connecting parts of the power unit is not required, and the energy efficiency of the engine can be improved.
For solving the aforesaid problem, the present invention employed the following constitution.
That is, an excavator in which an upper turning body is mounted on a lower traveling body, and an excavation attachment is provided on the upper turning body, comprising an engine as a power source, a generator driven by the engine, and a plurality of electric motors rotated by electric power supplied from the generator, whereby the electric motors as a driving source operate the lower traveling body, the upper turning body and each operating part of the excavation attachment.